It is almost a century since the great English philospher, Thomas Young, discovered in his own eye an error of refraction due to different refraction of different meridians. For half a century the knowledge thus obtained did not influence the practice of prescribing glasses. It was almost exclusively astronomers, scientists and physiologists, like Young,Airy,Stokes,Fick and others who studied this asymetry of refraction in their own eyes, measured it and tried to correct it. The first to have used cylindric glasses to this end was Airy. A new impetus was given to the study of this imperfection of the human eye by the invention of Helmholtz's ophthalmometer. With the aid of this perfect instrument, invented to determine the curvature of different surfaces, Helmholtz, Donders and their pupils, as Knapp, Woinow, Middelburgh, Mauthner, etc., studied in all directions the dioptric system of the eye. Fundamental points were